In modern digital video coding systems, two main modes are used to compress video signals: the INTRA mode and the INTER mode. In the INTRA mode, the luminance and chrominance channels are encoded by exploiting the spatial redundancy of the pixels in a given channel of a single image via transform coding. The INTER mode, exploiting the temporal redundancy between separate frames, relies on a motion-compensation technique that predicts a frame from one or more previously decoded frame(s) by encoding the motion of pixels from one frame to the other. In INTER mode the luminance and chrominance channels share the same motion description.
Usually a frame to be encoded is partitioned into independent blocks (macro block or pixel block) being compressed and encoded individually. In INTER mode each of the blocks are assigned one or several motion vectors and a prediction of the frame is constructed by displacing pixel blocks from past and/or future frame(s) according to the set of motion vectors. Finally, the difference between the frame to be encoded and its motion-compensated prediction, called the residual signal, is encoded in a similar way to the INTRA-mode by transform coding.
In MPEG terminology the INTRA mode corresponds to I frames, while the INTER mode corresponds to P and B frames. The coding efficiency of the INTER mode is much higher than the one of the INTRA mode, because it takes advantage of temporal prediction: much of the signal is contained in the prediction formed by motion compensation, and the residual signal has a smaller energy than the original signal. Because their encoding relies only on their own spatial redundancy, INTRA frames can be decoded independently from any other pictures, which is not the case for INTER frames, and are therefore inserted periodically in a bit stream. An INTRA-frame may be either a scene change frame, placed at the beginning of a new group of frames corresponding to a scene change where no temporal redundancy is available, or a refresh frame, placed in other locations where some temporal redundancy is available.
INTRA and INTER frames exhibit different coding artifacts, since the underlying encoding method is different. Throughout a homogeneous video sequence, the quality and artifacts of subsequent INTER frames tend to stabilize. However, if an INTRA refresh frame is encoded, all preceding artifacts, due to INTER coding, are erased, and new ones, due to INTRA coding, are introduced abruptly. Video quality is therefore disruptive at INTRA refresh frames, resulting in what is here called a flashing effect, also flickering and pumping effect are used in the literature. The flashing effect is especially visible in low motion sequences and at moderate or low bitrates, when coding artifacts become quite noticeable.
Various methods and devices for reducing this flashing effect exist. Some examples are disclosed in the following references: US2000710230574, US2007/0081591 and US2008/0025397.
The draw back with earlier methods is the added amount of calculations needed in terms of extra compression rounds. The present invention is directed towards an alternative method and device for reducing the flashing effect without adding extra rounds of compression.